The invention relates to a patient support system for irradiation therapy or treatment simulation, comprising a main supporting arm rotationally attached at one end to a structural support by a first support bearing so as to be rotatable about a first vertical axis fixedly located relative to the treatment isocentre, a patient support table top, interconnecting support means connecting the table top to the other end of the main supporting arm so that the table top can be displaced vertically and horizontally relative to the treatment isocentre and control means for controlling the displacement of the table top.
Radiation therapy involves directing a beam of high energy radiation such as hard x-rays or gamma rays from a suitable source, for example an isotope or a high energy x-ray tube using an electron accelerator, suitably a linear accelerator, at a selected region of the body of a patient in which malignant cells are present. A dosage is employed which is lethal to such cells, however, in order to minimise damage to other parts of the body, the dose applied to the surrounding tissue is reduced by rotating the direction of the irradiation beam about a central point called the treatment isocentre, which is located at or near the centre of the selected body region to be irradiated.
For this purpose a source of high energy radiation is mounted in counterbalanced manner on a gantry so as to be capable of rotation about a horizontal axis through the isocentre. The source is heavily screened to reduce generally emitted radiation to a reasonably safe amount and the emergent irradiation beam is limited by a diaphragm which defines the boundaries of the region of the patient to be irradiated, in a manner such that radiation is generally directed radially towards the isocentre. The source and gantry assembly is consequently very massive and is normally fixed to the structure of the associated building. This means that the isocentre is fixed in space within the treatment room. It is therefore a requirement for a patient support system that it should be capable of supporting and displacing a patient in an accurate and reproducible manner so that any body region to be irradiated can be located at the isocentre and positioned relative to the scanning arc of the radiation source so that irradiation may be applied to the patient along selectable directions of incidence.
A presently used form of patient support system of the kind referred to comprises a floor mounted relatively small turntable which is rotatable about a vertical axis usually through the isocentre and supports a radial arm extension the outer end of which is connected to a patient support table top by interconnecting support means in the form of a rotatably mounted pedestal base having an under table lift for vertical displacement and on the top of which is mounted a carriage assembly formed by a tandem arrangement of respective lateral and longitudinal horizontal displacement carriages, the table top being attached to the uppermost carriage. This form of patient support typically employs a form of scissors jack to provide the lift. Because of the relatively short stroke obtainable by such a jack and the need for a low minimum table height of about 70 cm for the convenience of patient access to the table top, this results in a relatively low maximum lifting height of about 120 cms. The maximum lifting height can be raised by the use of an underfloor pit. In some high energy installations the isocentre is even higher and a larger range of height adjustment can be provided using a hydraulic ram lift mounted on a more extensive turntable. This, however, requires the presence of a correspondingly deep underfloor pit to accommodate the ram housing resulting in higher installation costs and may not in some cases be structurally possible.
A further disadvantage of the above described patient support system concerns the carriage assembly for providing the horizontal displacement of the table top in two dimensions since the carriages and associated parallel rails render the assembly bulky and heavy, and tend to restrict access to the patient.
Our copending U.S. patent application Ser. No. 168,354, now U.S. Pat. No. 4,885,998 filed concurrently with this application, seeks to provide an improved patient support system for irradiation therapy which can reduce these difficulties and can be simple and convenient to operate in conjunction with computer controlled treatment or simulation equipment.
The patient support system disclosed therein comprises a main supporting arm rotationally attached at one end to a structural support by a first support bearing so as to be rotatable about a first vertical axis fixedly located relative to the treatment isocentre, a further supporting arm rotationally attached at one end to the other end of the main supporting arm by a second support bearing so as to be rotatable about a second vertical axis, vertical support means rotationally attached to the other end of the further supporting arm by a third support bearing so as to be rotatable about a third vertical axis, and a patient support table top attached to supporting carrier means, the vertical support means including means for locating and vertically displacing the supporting carrier means.
Control of the positioning of a patient support table top employed for irradiation therapy or treatment simulation, is usually carried out with reference to a set of cartesian coordinate axes in which the Z-axis is taken as being vertical and the origin is normally taken to coincide with the treatment isocentre.
In the course of treatment it is generally necessary to position and reposition a patient so that one or more selected regions of the body can be irradiated and, if necessary, displaced during irradiation, for example to treat an extensive region or to avoid irradiating an adjacent important healthy region while the irradiation beam source is being angularly displaced on the gantry.
In the case of the present table support system, measurements relating to the horizontal positioning of the patient are derived from respective angular displacement sensors coupled to respective shafts associated with relative angular displacements about the first, second and third vertical axes, and subsequent horizontal displacement of the patient support table top is effected by motor drives respectively arranged to modify these angular displacements. This introduces a difficulty in that, although a given arbitary combination of measured angular values can be accurately and rapidly converted by calculation into a corresponding pair of X- and Y-coordinate values relating to a reference point in a patient, it is not simple from a mathematical point of view to convert an arbitrary pair of X- and Y-coordinate demand values into a corresponding set of angular demand values for servo control of the motor drives about the three vertical axes, directly, with sufficient accuracy and speed to enable the patient support table top to be controlled in real time, while the use of a look-up table of sufficient accuracy would employ a prohibitive amount of memory.